Contact lenses having positional stabilization of the lens on the human eye correct, for example, axially-dependent ametropia of the eye such as astigmatism. The stabilization of the axial position of the contact lens prevents the rotational movement of the contact lens on the eye and assures the orientation of the contact lens even after a blinking of the eyelid. Various stabilization principles are known in order to obtain a positional stabilization of a contact lens on an eye.
The rotation of the contact lens on the eye can be prevented with a toric posterior surface if the corneal astigmatism of the eye is sufficiently great as disclosed in U.S. Pat. No. 4,787,732. However, this prerequisite is fulfilled only in the rarest of cases.
A very limited area of application is also provided for contact lenses which have an optical effect only for near vision. In this connection, reference may be had to German Patent 3,308,570 and U.S. Pat. Nos. 4,850,689 and 3,591,264.
Contact lenses having prismatic ballast have a stabilized position on the eye because of different thicknesses along the periphery of the contact lens and only a limited rotational movement occurs. Because of the increased weight of the lens in the lower region, the lens drops after each movement of the eyelid which pulls the lens out of the stabilized position and returns it to its use position as disclosed, for example, in U.S. Pat. No. 4,874,234. However, this thickening at the lower lens edge causes many wearers of contact lenses to experience a foreign-body sensation and the lens is therefore perceived as being disturbing. The same applies to contact lenses having a thickened edge in the lower lens region as suggested in German Patent 2,046,389 and German Utility Model Registration 7,034,876. A further disadvantage of contact lenses stabilized in this manner is that the oxygen permeability of a contact lens is greatly dependent on the thickness of the lens material in addition to the material itself and a high permeability with respect to oxygen must be required for a good compatibility of the contact lens on the eye.
German Patent 3,003,985 suggests that a stabilization can be achieved by permanently supporting the lens on the lower eyelid. However, the lower eyelid is subjected to a permanent irritation with the continuous support of the contact lens thereon with its entire weight. This irritation is perceived by the wearer of the contact lens especially when blinking.
The solutions for stabilization described above utilize static stabilization in order to suppress the movement of a contact lens on the eye of the wearer. However, there are also solutions for achieving stabilization which utilize dynamic stabilization. The dynamically stabilized contact lenses utilize the movement of both eyelids when blinking. In this way, the stabilization of the contact lens on the eye of the wearer takes place when the eyelids are closed.
A solution is known wherein the dynamic stabilization takes place by a reduction of the thickness at the outer surface of the lens at two regions which lie symmetrically to each other as suggested in U.S. Pat. No. 4,095,878 and British Patent 2,041,557. These regions are symmetrical to a center plane which is horizontal in the supporting position of the contact lens and these regions have a crescent-shaped configuration. In this way, the contact lens becomes thicker in the horizontal center plane than in the two crescent-shaped regions. When the eyelids are closed, the upper and lower eyelids of the eye glide over the contact lens and a pressure is applied to the contact lens because of the gradually increasing thickness of the contact lens in the crescent-shaped regions. This pressure is dependent upon the position of the contact lens on the eye and leads to an alignment of the contact lens. The alignment takes place with each blink of the lids whereas no stabilization takes place while the eye is open.
A further solution according to the dynamic stabilization principle is disclosed in U.S. Pat. No. 4,859,049 in which the contact lens is deliberately thickened in two outer regions in the horizontal center axis. These thickened regions lead to a targeted alignment of the contact lens when the eyelids are closed. Here too, the alignment takes place with each blink whereas no stabilization takes place when the eye is open.